1. Field of the Invention:
The present invention relates to a photodetector for detecting a scanning light beam in an optical scanner to detect the time to write information on a latent image carrier in a laser printer.
2. Description of the Relevant Art:
Generally, in laser printers, a scanning light beam that periodically sweeps in a main scanning direction is produced by a light beam scanning device and applied to a latent image carrier while the latent image carrier is being rotated in an auxiliary scanning direction, thereby forming a latent image two-dimensionally on the latent image carrier.
The light beam scanning device has a light deflector, such as a polygon mirror, for deflecting a light beam, which is emitted by a light source, in the main scanning direction. The scanning light beam thus produced is then applied to the latent image carrier. The scanning light beam starts being applied to the latent image carrier in timed relation to the detection, by a light beam detector of the scanning light beam, immediately prior to a scanning stroke thereof.
The light beam detector comprises a light-detecting element having a light-detecting surface directed toward the scanning light beam. The light-detecting element is electrically connected to an electric circuit in the light beam scanning device. In synchronism with the detection, by the light-detecting element, of the scanning light beam, the electric circuit in the light beam scanning device produces a horizontal scanning signal. Then, the light source of the optical scanner starts to apply a scanning light beam, which is modulated with the information of an image or the like to be printed, to the latent image carrier.
Therefore, the position where the desired information starts being written on the latent image carrier is determined by the exact time at which the horizontal synchronizing signal is produced.
Heretofore, it has been customary to cover the light-detecting element with a slit member that has a light-transmissive section and a light-blocking section, and attach the slit member to a baseboard on which the light-detecting element is mounted. With the light-transmissive section positioned over the light-detecting surface of the light-detecting element, the scanning light beam falls on only the stable-sensitivity area of the light-detecting surface.
Since the slit member is a separate member, however, the number of components of the light beam detector is relatively large. In order to direct the scanning light beam accurately to the stable-sensitivity area of the light-detecting surface, the slit member has to be secured to the baseboard with high positional accuracy. Consequently, the procedure for attaching the slit member to the baseboard with positional accuracy with respect to the light-detecting element has been cumbersome.
The dimensional accuracy of the width of the light-transmissive section of the light-detecting surface along the main scanning direction directly affects the duration of the output signal from the light-detecting element. Inasmuch as the slit member is generally molded of synthetic resin, it is difficult to increase the dimensional accuracy of the width of the light-transmissive section. Therefore, the duration of a pulse signal which is produced by comparing the output signal from the light-detecting element with a threshold signal tends to be unstable, adversely affecting the accuracy of the position where the desired information to be printed is written on the latent image carrier.